1. Field of the Invention
This invention relates generally to optical data carriers and reading devices therefor. The invention can be applied, for example, to an optical data carrier of the kind having diffraction patterns arranged along at least one data track, wherein each diffraction pattern represents a bit sequence with n bits and is selected in accordance with the bit sequence to be represented from a set of m different diffraction patterns. The invention can also be applied, for example, to a reading device for reading such a data carrier and a key provided with such a data carrier.
2. Description of the Prior Art
Such data carriers are suitable for example for the storage of text, video and audio information or other items of information such as for example biometric information which are to be available in the form of ROM data. Known data carriers of that kind are CDs (compact discs). Furthermore, such data carriers are also suitable for use as value-bearing cards which are difficult to copy or forge such as, for example, charge cards for the payment of telephone calls using the public telephone system, in which a large amount of information is to be stored in a restricted space.
It is known from European application EP 376 673, in the case of a CD, to arrange bar-like pits along the data tracks in different orientations, each orientation being associated with a different bit sequence. Photodetectors are provided for reading the bit sequence, the number of photodetectors being twice as great as the number of different orientations of the pits. In the best case the number of photodetectors provided is equal to the number of different orientations plus one.
An optical data carrier of the kind set forth in the classifying portion of claim 1 is known from European Application EP 613 126. Cells provided with diffraction gratings are present on the data carrier. Each cell is subdivided into a number n of surface portions, wherein associated with each surface portion is a diffraction grating involving different parameters such as line spacing and/or azimuth. Each surface portion diffracts an impinging reading light beam as beam portions in different directions if it represents a bit of the value "1", and it diffracts no light if it represents a bit of the value "0". The bit sequence stored in a cell is thus read out simultaneously. As the surface portions are arranged in side-by-side relationship, the surface area demanded by a bit sequence is relatively large in comparison with a pit representing a single bit.
Thermoplastic materials are also known which can be embossed with microscopically fine relief structures. The relief structures can be locally and irreversibly altered by the application of heat energy.